Last week Al Kossow posted some new sources for XXDP tests on Bitsavers. Included are the sources for the MMU tests for 11/34. I tried to run those, and was surprised by a list of error messages… Turns out, there were a number of issues left in my MMU implementation, that none of the other tests I’ve used so far had picked up. To be more specific, FKTA, FKTB, and FKTC all three complained about issues that FKTH, KKTA, KKTB, and ZKDK let pass…

  • mtpi worked in the wrong order. Because the instruction follows the more or less regular destination pattern, I did the address calculation for the destination first, and only after that the special part for mtpi – finding the implied operand from the stack, and popping the stack in the process. That gives wrong results if the stack pointer is updated in the destination address calculation… An obscure case, probably, and I’m not at all sure that this exact process is used by all PDP models. Nevertheless, there is a fix.
  • the a and w bits in the pdr registers should be reset on a write. However, my implementation only did that for word or even byte writes – not for odd byte writes.
  • and probably the most promising of all, the a and w bits in the pdr registers should not be set if the access was aborted by the mmu.

I had some hope that these fixes might also have an impact on the still mysterious problem in booting RSX-11M-Plus. No luck though… Still, it never ceases to amaze me how good and how devious some of these test programs are, it really sometimes takes hours to find out what a test does, and how to make the cpu and mmu work as it was intended. And, obscure though these issues may seem, they may in some way impact how some old software runs on the VHDL.
Anyway, I’m running a number of regression tests now, and will post the latest versions some time later. And maybe play with some of the other tests as well – there could still be more to find there.
And of course a big thankyou to Al!

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